1. Field of the Invention
The present invention relates to an image pickup lens system and particularly, to an image pickup lens system which is comprised of three lenses and used in an image pick-up device (e.g., an image-inputting CCD camera) utilizing a solid image-pickup element such as CCD, CMOS and the like mounted on a potable computer, a visual telephone and the like, and which can be reduced in size and weight.
2. Description of the Related Art
In recent years, multimedia have been developed remarkably, and for example, the demand for a camera utilizing an image pickup element such as CCD, CMOS and like, e.g., a CCD camera, mounted on a portable computer, a visual telephone and the like, is being increased remarkably. Such a CCD camera is required to be mounted in a limited space and for this reason, it is desired that the CCD camera is small in size and lightweight. Therefore, it is also desired that an image pickup lens system used in such a CCD camera is likewise small in size and lightweight.
A one-lens system using a single lens or a two-lens system using two lenses is conventionally used as such an image pickup lens system.
However, such a lens system is convenient extremely for reducing the size and weight of the lens, but recently suffers from a problem that it is not suitable for increasing the picture quality and the resolution required for the image pick-up lens system.
For this reason, it is a conventional practice to use a three-lens system using three lenses, thereby providing increases in picture quality and in resolution.
Such a three-lens system has a long history in the field of a silver-salt photograph camera, and optical lens systems of various arrangements have been developed.
In the lens system in the silver-salt photograph camera, however, the diameter of a lens is larger and moreover, the focal length is larger. Therefore, even if the lens is utilized in a shape as it is and in a reduced size as an image pickup lens for an image pickup element, many disadvantages are encountered, for example, the thickness of the center and the flange portion of the lens is extremely small; an exit pupil is too close to an image surface; and a back focal length is reduced. In this way, such lens cannot be utilized as it is.
Therefore, a three-lens image pickup lens system exclusively used in an image pickup element has been conventionally developed. One example of such image pickup lens system is a lens system comprising a lens having a first convex-shaped face adjacent an object and having a negative power, a lens having a negative power and a lens having a positive power, which are arranged sequentially from the side of the object.
However, such conventional image pickup lens system, because of the first face of the first lens formed into a convex face, suffers from the following problems: A large back focal length cannot be ensured, and various aberrations including mainly a chromatic aberration cannot be corrected appropriately. Further, it is difficult to ensure a large distance from an exit pupil to an image surface, and it is impossible to ensure a telecentric property.
Accordingly, it is an object of the present invention to provide an image pickup lens system in which a large back focal length can be ensured, while ensuring a large angle widening and maintaining a desired optical performance, and a high telecentric property can be ensured and moreover, various aberrations can be corrected satisfactorily, and which can be produced easily.
To achieve the above object, according to the present invention, there is provided an image pickup lens system comprising a first lens which has a concave face formed on the side of an object in the vicinity of an optical axis, and which has a positive power, a diaphragm, a second lens having a negative power, and a third lens having a positive power, said first lens, said diaphragm, said second lens and said third lens being arranged sequentially from the side of the object.
With the above feature, the first lens has the concave face formed on the side of the object in the vicinity of the optical axis, and has the positive power. Therefore, a sufficient back focal length can be ensured, while maintaining a desired optical performance and ensuring a large angle-widening, and a distance from an image surface to an exit pupil can be ensured to ensure a high telecentric property. Moreover, the entire optical system can be reduced in size and can be produced easily. An unnecessary light beam of an off-axial flux causing a flaring and the like can be corrected or removed effectively by disposing the diaphragm between the first and second lenses, as compared with a case where the diaphragm is disposed at a location closer to an object from a first lens.
According to the present invention, the radius |r1| of curvature of the first face on the side of the first lens at its center is equal to or smaller than a focal length f1 of the entire optical system.
With such feature, the satisfaction of the above condition ensures that a large back focal length can be ensured, and a further increase in angle of view and a reduction in focal length can be realized, while providing a reduction in size of the entire optical system.
According to the present invention, the diaphragm is disposed at a location closer to the first lens from an intermediate point between the first and second lenses.
With above feature, the diaphragm is disposed at the above-described location and hence, an increase in size of each lens, particularly, the third lens can be prevented, while ensuring a sufficient distance from an image pickup surface to an exit pupil.
According to the present invention, the first face of at least the second lens is formed into an aspherical shape.
With the above feature, various aberrations can be corrected affectively, because the first face of at least the second lens is formed into the aspherical shape.
According to the present invention, the second lens is a lens formed into a concave face on the side of the object in the vicinity of an optical axis and having a negative power.
With the above feature, the face (first face) of the second lens on the side of the object is the concave face and hence, various aberrations including mainly an axial chromic aberration can be corrected satisfactorily, and an exit pupil can be positioned at a point far away from an image surface, while shortening the entire length of the optical system, thereby ensuring a high telecentric property. Further, the second lens can be formed at a high eccentricity resistance and produced easily.
According to the present invention, the second lens is a lens, which has a convex face formed on the side of the object in the vicinity of an optical axis and which has a negative power.
With the above feature, the face (first face) of the second lens is the convex face and hence, a further reduction in size of the entire optical system can be provided, while ensuring a high telecentric property to a certain extent, as compared with a case where such a face is a concave face.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.